1. Field of the Invention
The instant invention relates to methods for modeling and building drill bits and, more particularly, to such a method in which a geometrical model of the bit can be used to calculate drill bit cutter forces for a given set of drilling conditions.
2. Setting of the Invention
Solid body drill bits of the type not having rolling cones usually include a bit body, typically having a tapered lower end and an upper end, which is connectable to a drill string for rotating the bit and applying weight thereto during drilling. These bits include a plurality of cutters each of which includes a substantially planar cutting surface. The cutters are usually mounted on the tapered lower portion of the bit with a few being mounted on the side adjacent the tapered portion.
In order to drill through a subterranean formation material, a penetrating force, which is produced by the weight of the drill string, must be applied to the drill bit in order to embed the cutters into the rock and to keep the cutters embedded during drilling. Once the cutters are embedded, a cutting force must be applied thereto in order to cut rock from the formation. The cutting force is applied by rotating the drill string.
For a given rock strength, the drill bit rotation rate and bit penetration rate require a corresponding penetration force and cutting force on the cutters. In general, as rock strength and rotation and penetration rates increase, the required penetrating and cutting forces increase. The magnitude and direction of penetrating and cutting forces applied to the cutters is also dependent upon the angular orientation of each cutting surface. Since the cutters are usually mounted on a tapered lower surface of the drill bit, both the penetrating and cutting forces acting on the cutters include a radial component relative to the drill bit longitudinal axis. The sum of the radial components of the cutting and penetrating forces are known as the "imbalance force." This imbalance force tends to push the bit into the wall of the hole as the bit rotates. A bit with a large imbalance force drills more slowly and does not last as long as a bit with a relatively small imbalance force. It is thus desirable to design drill bits having minimal imbalance forces.
Past drill bit designers have used equations from which the penetrating and cutting forces can be determined for a given penetration or depth of cut (which is a function of the rotation and penetration rates), rock strength and cutting face positions relative to the longitudinal axis of the drill bit. Bits in the past have been designed to produce an imbalance force (the sum of the radial components of the cutting and penetrating forces) of zero. However, because of the manufacturing process for such bits, the bit as manufactured often in fact have a relatively large imbalance force.
Manufacturing drill bits typically involves brazing a polycrystalline diamond compact (PDC) directly to the drill bit body or to a stud which is press fitted into the body. The brazing and press fitting processes do not permit accurate positioning of the PDC cutting surface and thus the positions of the PDC cutting surfaces as manufactured may vary from the positions as designed thereby introducing a significant imbalance force. The imbalance force causes the bit to have a shorter life and a slower rate of penetration than a bit with a nonexistent or relatively small imbalance force.
It would be desirable to have a method for calculating the cutter forces and imbalance force for a selected drill bit under a given set of drilling conditions prior to actually using the bit. Such a method can be used as a quality control to select bits having the lowest imbalance forces arising therein under given drilling conditions. It would be desirable to utilize a method of manufacturing drill bits which minimizes the imbalance forces arising in the drill bits when drilling.
There exists a need for accurately calculating the cutter forces and imbalance force which will act on a selected drill bit for a given set of drilling conditions.
There exists a need for a method of manufacturing a drill bit which, will minimize the imbalance which arises in the bit during drilling.